One thing that can be done with a metal ball that Nitrile can't do, is to whack one with a hammer into the seat so it deforms to fit the ball accurately. Replace the whacked ball with a new one because it probably squished out of shape and is no longer a sphere.
Beware – I've never tried it!
Dave
Edited By SillyOldDuffer on 20/05/2023 19:18:24